Method for treating chromium-containing baths

ABSTRACT

In accordance with certain of its aspects, this invention relates to a novel apparatus and to the process for treating a bath containing trivalent chromium to convert said trivalent chromium to hexavalent chromium which comprises maintaining in said bath an electrolyzing cathode of predetermined area, the surface of which is in intimate electrical contact with a material in a low hydrogen overvoltage state selected from the group consisting of etched iron compound, platinum, palladium, rhodium, iridium, and nickel; maintaining in said bath an electrolyzing anode having a surface area at least equal to the area of said cathode; and electrolyzing said bath between said electrolyzing cathode and said electrolyzing anode at an electrolyzing anode current density of at least 0.3 amperes per square decimeter whereby said trivalent chromium in said bath is converted to said hexavalent chromium.

United States Patent [72] Inventors [22] Filed Jan. 26, 1966 [45] Patented Oct. 26, 1971 [73] Assignee M & T Chemicals Inc.

New York, N .Y.

[54] METHOD FOR TREATING CHROMIUM- CONTAINING BATHS 2 Claims, 1 Drawing Fig.

[52] U.S.Cl i. 204/51, 204/89, 204/232, 204/252 [51] Int. Cl C23b 5/06, B01k1/00,C01g 37/00 [50] Field of Search 204/51,

Society, vol. 34, pp. 228- 240, 1947.

Tufanov, D. G., Vestnik Inzhenerovi Tekh. No. 8, pp. 268- 270, 1946.

Morisset, Paul et al., Chromium Plating, pp. 47l 473, 1954.

Primary Examiner-G. L. Kaplan AttorneyCarl G. Seutter ABSTRACT: In accordance with certain ofits aspects, this invention relates to a novel apparatus and to the process for treating a bath containing trivalent chromium to convert said trivalent chromium to hexavalent chromium which comprises maintaining in said bath an electrolyzing cathode of predetermined area, the surface of which is in intimate electrical contact with a material in a low hydrogen overvoltage state selected from the group consisting of etched iron compound, platinum, palladium, rhodium, iridium, and nickel; maintaining in said bath an electrolyzing anode having a surface area at least equal to the area of said cathode; and electrolyzing said bath between said electrolyzing cathode and said electrolyzing anode at an electrolyzing anode current density of at least 0.3 amperes per square decimeter whereby said trivalent chromium in said bath is converted to said hexavalent chromium.

PATENTEIJnm 2s IHTI 3,616,304

la l8 l5 I6 I M W L L/l/W I I I I I I I I I I /4 l3 /4 /2 I I I I I I I /9 IL/ I I I I I I /O I I I I INVIiNTURS PAM DEV 4950/ BY Polwuo Dow METHOD FOR TREATING CHROMIUM-CONTAINING BATHS This invention relates to the treatment of chromium-containing baths. More particularly, this invention relates to a method of reducing or lowering the concentration of trivalent chromium in chromium-containing baths, including chromium-electroplating baths.

As is well known to those skilled in the art, chromium may be deposited from electroplating baths containing chromic acid, CrO together with sulfate and various other materials. Chromium as deposited may be obtained in very thin or decorative thicknesses of up to about 5 microns, or in hard chromium industrial deposits which may have a thickness of as much as 2,500 microns, i.e. 2.5 mm. During normal electrodeposition of either hard or decorative chromium plate, deposition of chromium metal may be accompanied by an increase in the concentration of trivalent chromium, Cr. This buildup ofthe concentration of trivalent chromium may be increased due to dissolved or dragged in impurities, such as copper, iron, or nickel.

As the concentration of trivalent chromium increases in the bath, many undesirable and disadvantageous features become observable. Baths containing rather appreciable amounts of trivalent chromium (i.e. trivalent chromium in amounts typically 5 to 8 g./l. or more) possess decreased throwing power and low-current density areas may not be satisfactorily plated. This loss of throwing power is particularly undesirable in the case of decorative plating.

Presence of trivalent chromium in the noted concentrations increases the resistance of the plating bath. Accordingly, the throwing power is reduced and more total power may be required to achieve the desired current density necessary for chromium plating. Furthermore, the higher power input required raises the temperature of the bath which increases the cooling requirements needed to maintain the bath at desired temperature.

As is also known, presence of such high levels of trivalent chromium in hard chromium-plating baths causes the deposited chromium to pit and tree to a much greater degree than would otherwise occur. Furthermore, the plating speed and the efficiency of the bath decreases; and thus it requires longer times, at the same current, to achieve desired plate thickness.

ln accordance with prior art techniques when the trivalent chromium concentration ofa chromium plating bath increases much above the level of5 g./l., and certainly when it increases up to 2-3 times this amount, the bath may be treated to lower the level of trivalent chromium. Commonly this may be effected by pumping the solution out of the bath, cooling it, diluting or bleeding it, and thereafter passing the solution through an appropriate ion-exchange system wherein trivalent chromium is removed. The solution so treated could then be u sed as a maintenance solution ofthe bath.

It is an object of this invention to provide a novel technique for lowering the concentration of trivalent chromium in chromium-containing baths typified by chromium-electroplating baths. Other objects of this invention will be apparent to those skilled in the art from inspection of the following description.

in accordance with certain of its aspects, the process of this invention for treating a bath containing trivalent chromium to convert said trivalent chromium to hexavalent chromium comprises maintaining in said bath an electrolyzing cathode of predetermined area, the surface of which is in intimate electrical contact with a material in a low hydrogen overvoltage state selected from the group consisting of etched iron compound, platinum, palladium, rhodium, iridium, and nickel; maintaining in said bath an electrolyzing anode having a surface area at least equal to the area of said cathode; and electrolyzing said bath between said electrolyzing cathode and said electrolyzing anode at an electrolyzing anode current density of at least 0.3 amperes per square decimeter whereby said trivalent chromium in said bath is converted to said hexavalent chromium.

This invention may be particularly useful in chromium-plating systems and for the purpose of convenience reference may be hereinafter made to such systems. Chromium-plating systems with which the process of this invention may find particular advantage may include decorative or hard chromium systems. In a typical decorative system, the bath may contain typically 200-400 g./1., say 250 g./l., chromic acid CrO,, and 0.5-5.0 g./l., say 1.0 g./l., sulfate ion together with other desired ingredients typically 1.0-6.0 g./l., say 2.0 g./l., of silicofluoride ion (as potassium silicofluoride). Chromium plating in such a decorative system may be carried out at 35-90 C., typically 40-60 C. for l-15 minutes, say 5 minutes, at a cathode current density of 5-90, and typically 12-45 amperes per square decimeter (asd), and at an anode current density of about the same as the cathode current density, to produce a plate of chromium having a thickness of 0125-50 microns say 0.75 microns. In such a system, it may be common to use a lead anode, including lead dioxide anode.

Typically in a hard chromium system, the bath may contain -400 g./l., say 200 g./l. chromic acid (CrO and 0.5-5.0 g./l., say 1.5 g./l., sulfate ion (50;) together with other desired ingredients typically 1.0-6.0 g./l., say 2.0 g./l., of silicofluoride ion (added as potassium silicofluoride). Chromium plating in such a hard or industrial chromium plating system may be carried out at 3590 C., say 55 C., for 01-24 hours, say 2 hours, at a cathode current density of 5-150 asd., and typically 50 asd. and at an anode current density of about 5-150 asd., to produce a chromium plate having a thickness of say 0.003 inch (75 microns). In such a system it may be common to use a lead anode including lead dioxide anode.

The basis metal cathodes which may be plated in such processes may be those metals on which a chromium plate is desired. Typically such metals may be iron or cast iron, or iron alloys including steels such as stainless steels, low-carbon steels, nickel steels, chromium-nickel steels, etc., particularly when these metals are in bright, solid, highly polished condition. Nonferrous metals including nickel, copper, brass, zinc, and aluminum may be plated. it is common to first plate these metals with a layer of nickel frequently preceded by a layer of copper.

When chromium plating baths useful for hard or decorative plating are made, the concentration of trivalent chromium may be negligible, typically less than about 1.0 g./l. As the solution is electrolyzed, the hexavalent chromium present may be reduced to trivalent chromium by the action of reducing agents inadvertently introduced into the bath, by drag-in, by contaminants, or by the dissolution of metals, or through the reducing action occurring at the cathode. In this manner, the concentration of trivalent chromium in decorative baths may increase to 5.0l5.0 g./l., say 10.0 g./l. and in hard chrome baths to 10.0-25.0 g./l., say 15.0 g./l., at which point the baths may undesirably be characterized by increased resistance, reduced throwing power, loss of plating speed, and increased tendency to produce pitted and rough deposits.

The concentration of trivalent chromium may be lowered by practice of this invention by maintaining in a chromium electroplating bath, preferably the same electroplating bath as that in which the electroplating procedure is practiced, an electrolyzing anode and an electrolyzing cathode in addition to the plating anode and the plating cathode normally present. The electrolyzing cathode should be a cathode of predetermined area and its surface should be in intimate electrical contact with a material in low hydrogen overvoltage state selected from the group consisting of etched iron compound, platinum, palladium, rhodium, iridium, and nickel. The additional or electrolyzing cathode which may be used in practice ofthis invention according to one embodiment may be formed of a material selected from the group consisting of platinum, palladium, rhodium, iridium, and nickel, having a surface thereon of metal in a low hydrogen overvoltage state. In a preferred embodiment however the electrolyzing cathode may be any basis metal including iron, such as cast iron and steel. including stainless steels, low carbon steels, nickel steels, chromium steels, chromium-nickel steels, etc. or nonferrous metals such as nickel, copper, brass, zinc, aluminum, etc. This cathode may comprise a metal the surface of which is placed in intimate electrical contact e.g. coated with a material such as etched iron compound, platinum, palladium, rhodium, iridium, nickel, etc., in a low hydrogen overvoltage stage. Such materials in their low hydrogen overvoltage form may be commonly characterized by finely divided surface condition. Low hydrogen overvoltage materials are described on page 116 of Reference Electrodes by lves and Janz (1961) Academic Press, New York. An electrolyzing cathode such as that described herein bearing a surface in the low hydrogen overvoltage state remains free of chromium plate in a chromium-electroplating bath. A particularly preferred cathode may be cast iron, the surface of which is a chemically etched compound in a low hydrogen overvoltage state. The etching may typically result from the action of acid on an iron compound such as iron carbide, iron nitride, or iron phosphide.

These surfaces may be placed in intimate electrical contact with the electrolyzing cathode (which preferably has been cleaned or prepared) in a number of ways. One highly convenient method for applying the surface to the cathode is by deposition, typically electrical, chemical, or immersion deposition, from a solution containing the ions of a particular metal. This coating or deposition may be accomplished e.g. by spraying, contacting, brushing, dipping, electroplating, immersion plating by the process similar to electroless nickel plating technique, etc. when the ions of the metal come into contact with the cathode, the former may be chemically reduced to font: a deposit of the metal on the surface of the basis metal; and such deposits may typically be in the form of finely divided metal, e.g. black metal, such as platinum black, palladium black, or black nickel.

Compounds of the metal including salts, acids, etc. may also be employed in practice of this invention. For example, chloroplatinic acid solutions may be employed as a source of platinum ions. Similar equivalent metal compounds may be employed. when the metal may exist in more than one oxidation or valence state, any of these may generally be employed. Typical ionic metal compounds which may be used to obtain metal deposits which may preferably find use in the practice of this invention include palladium dichloride, chloroplatinic acid, platinum chloride, platinum diamine dinitrite, potassium chloroplatinate, potassium chloroplatinite, tetrammine platinous choride, tetrammine platinous fluoride, palladium nitrate, rhodium chloride, iridium tetrachloride, chloroiridic acid, etc. Other deposits may be employed. The preferred compounds may include palladium chloride, rhodium chloride, and chloroplatinic acid in aqueous solutions. Solutions of these in organic solvents such as ethanol, propanol, acetone, benzaldehyde, ether, etc. may be employed.

in a preferred embodiment when it is desired to treat a cathode such as a steel rod or strip to be used in practice of this invention, the cathode may be coated, with e.g. platinum, by brushing or by treating the surface in question as by dipping the piece into an aqueous (or a nonaqueous) solution of a complex salt such as a platinum salt complex, Le. containing chloride, PtClZ which may be present as an alkali metal salt thereof, e.g. NruPtCl Typically an aqueous solution containing at least about 0.01 g./l. up to the limit of solubility, and say 3 Mi. ol complexed platinum metal added as chloroplatinic acid, and 4-350 g./l., preferably 20-80 gJl. cl sodium chloride, may be used. it is a particular feature of this invention that the presence of sodium chloride in the noted platinum solution permits a high degree of control of rate of deposition of platinum; it also imparts to the deposit a characteristic dark color which facilitates application; and it further imparts a greater degree of adhesion of the platinum. The basis metal may be printed with this solution which may be allowed to stay thereon for typically l5-30 seconds. During this period. finely divided platinum metal may cost the basis metal.

when the electrolyzing cathode has acquired such a coating, the solution may be removed as by washing or rinsing with water, the so-treated cathode may then be ready for further use.

in another embodiment, the electrolyzing cathode may be dipped into a solution of palladium ion e.g. palladium chloride (having a concentration of at least about 0.1 gJl. up to 25 gJl. and preferably 4 g./l. and immersed for at least about l2 seconds; the cathode may then be removed from the solution, rinsed, cleaned, and thereafter used as hereinafter set forth.

in accordance with another embodiment of this invention, the cathode (typically of steel) may be treated by brushing or spraying thereon a solution of the particular metal salt, and allowing the solution to remain in contact with the surface of the cathode for a period of time sufficiently long (at least 1 second and typically 1-2 seconds) to assure adequate reaction.

in accordance with certain preferred aspects of this invention, the cathode may be made from a piece of cast iron, including high-carbon cast iron alloy, and case hardened or nitrided steel, and the low hydrogen overvoltage surface may be formed by etching, e.g. by pickling the surface of the cathode in mineral acids, preferably in dilute hydrochloric acid. An etched cast iron cathode has a surface in a low hydrogen overvoltage state and resists chromium plating, apparently due to the action of acid on iron compounds which may be present as iron carbide, iron nitride or iron phosphide.

in accordance with another aspect of this invention, the low hydrogen overvoltage metal, which is to be placed in intimate electrical contact with the surface of the electroiyzing cathode, may be provided by means of a thin foraminous sheet, including mesh, ex anded metal, perforated metal, etc. This may be placed in intimate electrical contact with, and preferably positioned immediately adjacent to and electrically connected to the electrolyzing cathode surface areas. The foraminous sheet itself may be made of a metal of low hydrogen overvoltage or it may be a basis metal (e.g. steel) coated with a low hydrogen overvoltage metal such as platinum, palladium, etc. in the manner noted supra. The electrolyzing cathode may preferably be overlaid with and contiguous to the thin foraminous sheet.

The quantity of the low hydrogen overvoltage metal which may be employed may be very small. Larger amounts may be employed, but it may be found that such larger amounts while producing thicker surfaces, may not impart any appreciable improvement. Although the preferred amount of metal may be essentially the amount required to form a monatomic layer thereof on the cathode, it may be found that a continuous monatomic layer need not be formed. instead the low hydrogen overvoltage metal may be present in the form of discrete islands and a considerable portion of the treated area of the cathode may superficially appear to be uncovered. Nevertheless, the scattered islands of low hydrogen overvoltage metal may be found to be entirely effective for the purposes of the invention, i.e. in the prevention of chromium deposition on the entire area of the electrolyzing cathode used; the low hydrogen overvoltage metal also facilitates harmless hydrogen evolution on the electrolyzing cathode.

in practice of the process of this invention, there may be employed an apparatus for electroplating comprising an aqueous bath containing chromium and having maintained therein an electrolyaing cathode of predetermined area the surface of which is in intimate electrical contact with a material in a low hydrogen overvoltage state selected from the group consisting of etched iron compound, platinum, palladium, rhodium, iridium, nickel; an electrolyzing anode having a surface area which may be at least equal to the surface area of said electrolyzing cathode; a plating cathode and a plating anode; an electrolyzing current source connected to said electrolyzing cathode and said electrolyzing anode; and a plating current source connected to said plating cathode and said plating anode.

in the above-described apparatus, the electrolyzing cathode in intimate contact with low hydrogen overvoltage material and the electrolyzing anode may be positioned at any point in the electroplating bath, although preferably it will be out of direct interposition between the plating anode and the position where articles are ordinarily chromium plated (i.e. the plating cathode). Typically, the electrolyzing cathode may, for example, be positioned OK to one side of its anode, or behind it and between it and the wall of the tank. Optionally a porous partition e.g. of porous or foraminous material or of plastic cloth such as polypropylene, polyvinyldichloride, polytetrafluoroethylene, etc., may be positioned in the bath between the plating and the electrolyzing electrical circuits. Preferably the electrolyzing anodes may possess a total area 1-10 times larger than the area of the electrolyzing cathode.

In the FIGURE there is shown one specific embodiment of this aspect of the invention wherein chromium plating bath contains plating anode 11 and plating cathode 12. The plating anode 11 and plating cathode 12 are connected by conductors 15 and 16 to a plating current source not shown. In this embodiment, the bath also contains the additional or electrolyzing anodes 14 and the additional or electrolyzing cathode 13. The electrolyzing cathode 13 and the electrolyzing anodes 14 are connected by conductors 17 and 18 to an electrolyzing current source, not shown. There is also present a porous plastic partition 19 between the plating and the electrolyzing electrodes.

The anodes which may be employed in the invention may be of platinum, gold, lead, including lead dioxide, etc., most preferably of lead or lead dioxide. When plating electrodes are employed in baths containing electrolyzing electrodes, preferably the plating anode and the electrolyzing anode are ofthe same material, e.g. lead or lead alloy, or lead dioxide.

In practice of the process of this invention for converting trivalent chromium to hexavalent chromium, a current may be passed between the electrolyzing cathode 13 and the electrolyzing anodes 14 in the figure. Preferably the current density on the electrolyzing anode will be more than about 0.3 asd., and typically 1.0-5.0 asd. The trivalent chromium may be generated during chromium plating as chromium is deposited on plating cathode 12 as current passes between the plating anode 11 and the plating cathode l2. Trivalent chromium may be reoxidized to hexavalent chromium at the electrolyzing anodes 14. During oxidation of trivalent chromium at the electrolyzing anodes 14, it may be noted that the presence of the low hydrogen overvoltage metal in intimate electrical contact with the surface of the electrolyzing cathode 13 permits hydrogen evolution without deposition of any chromium metal on the surfaces of the electrolyzing cathode 13. Uniformity may be established throughout the bath by agitation of the solution as by external agitation; more commonly however, the normal agitation occurring within the chromium plating bath (because of the liberation of gases such as hydrogen and oxygen) may be sufficient to provide uniformi- I".

In accordance with certain of its aspects the process of this invention may be performed in a process of treating a chromium-plating bath containing trivalent chromium to convert trivalent chromium to hexavalent chromium, which comprises maintaining a plating anode and a plating cathode in a bath containing hexavalent chromium; plating said plating cathode by passing a plating current between said plating anode and said plating cathode whereby chromium is deposited onto said plating cathode and trivalent chromium is formed in said bath; maintaining in said bath an electrolyzing cathode of predetermined area the surface of which is in intimate electrical contact with a material in low hydrogen overvoltage state selected from the group consisting of etched iron compound, platinum, palladium, rhodium, iridium, and nickel and an electrolyzing anode having a surface area at least equal to the area of said electrolyzing cathode; and electrolyzing said bath between said electrolyzing cathode and said electrolyzing anode at an electrolyzing anode current density of at least 0.3 amperes per square decimeter whereby said trivalent chromium in said bath is converted to said hexavalent chromium.

In the preferred embodiment, the electrolyzing cathode and anode system may be operated during the chromium-plating operation. When the preferred electrolyzing anode current density is maintained upon the electrolyzing anode system having designated surface area, it will be found that the concentration of trivalent chromium may be maintained at a level less than 5 g./l. and typically less than about 3 g./l. In the preferred embodiment, it has been found that the electrolyzing system may maintain the level of trivalent chromium at the very low level at which it may be in a fresh unused bath.

Practice of this invention may be observed from the following examples. All parts, except as otherwise indicated, are by weight.

EXAMPLE l In this example, a chromium plating bath may be made containing 23.7 parts of chromic acid, 0.72 parts of strontium sulfate, 1.50 parts of potassium silicofluoride, 3.15 parts of potassium dichromate, and 0.63 parts of strontium chromate in parts total of aqueous solution. This freshly made plating bath may be analyzed by standard analytical techniques and found to contain substantially no parts of trivalent chromium. In order to artificially generate trivalent chromium, 0.53 parts of cane sugar may be added to the solution as a reducing agent; after this addition the bath may be analyzed and found to contain 21.4 parts (214 g./l.) of chromic acid and 1.32 (13.2 g./l.) parts of trivalent chromium.

The bath may then be electrolyzed with two lead anodes having a total surface area of 0.6 square decimeters and a steel cathode which has a surface area of0.6 square decimeters and which prior to use has been dipped into 1,000 parts total of an aqueous solution containing 4 parts of palladium chloride, 20 parts of sodium chloride, and sufficient hydrochloric acid to lower the pH to 1.5 to plate on its surface a low hydrogen overvoltage layer of palladium. The cathode may be left in the solution for 15 seconds, removed and washed thereby producing an electrolyzing cathode.

A current may be passed between the electrolyzing cathode and the anodes at a cathode current density of 31 asd. and an anode current density of 31 asd. for 24 hours. After electrolysis for 24 hours the solution may be analyzed for concentration of trivalent chromium and for chromic acid. It may be found that this solution to which cane sugar had been added prior to electrolysis and which contained 21.4 parts (214 g./l.) of chromic acid and 1.32 parts (13.2 g./l.) of trivalent chromium may be found after 24 hours ofelectrolysis to contain 23.2 parts (232 g./l.) of chromic acid and 0.41 parts (4.1 g./l.) of trivalent chromium.

Thus it will be observed that in this particular embodiment, the concentration of trivalent chromium may be reduced from 13.2 g./l., in a bath containing a substantial amount of trivalent chromium, to 4.1 g./l., a decrease by a factor of about 69 percent, whereas in an ordinary chromium-plating system with an anode to cathode ratio of 1:1 during chromium plating, the trivalent chromium concentration would remain at least about 13.2 g./1.

EXAMPLE 2 In this embodiment of the process of the invention, the process of example 1 may be followed except that the electrolyzing cathode employed in this example may have a surface area one-half as large as that of example 1, and thus the ratio of anode surface area to cathode surface area may be 2:]. The initial Solution may have concentration of 23.7 parts (237 gJI.) of chromic acid. After addition of the cane sugar, the solution may contain 20.2 parts (202 g./1.) of chromic acid and 1.9 parts (19 g./1.) of trivalent chromium. Afier treatment by the process of this invention for 24 hours the solution may contain 23.4 parts (234 g./!.) of chromic acid and 0.25 parts (2.48 g./1.) of trivalent chromium. It will be obvious that the process of this invention permits decrease in the amount of trivalent chromium present by a factor of 87 percent.

EXAMPLE 3 In this example the process of example 1 may be followed except that the electrolyzing cathode employed may be of high-carbon cast iron alloy having the iron compound surface previously etched in dilute hydrochloric acid and having a surface area of 0.3 square decimeters. (Thus the ratio of anode surface area to cathode surface area may be 2:1 After the addition of cane sugar to the initial solution having a concentration of 23.7 parts (237 g./1.) of chromic acid, the solution may contain 20.2 parts (202 g./|.) of chromic acid and 1.9 parts (19 g./1.) of trivalent chromium. After treatment by the process of this invention over a period of 12 hours, the solution may contain 23.1 parts (231 g./l.) of chromic acid and 0.45 parts (4.5 3.11.) of trivalent chromium, representing a decrease in the amount of trivalent chromium by a factor of 76 percent.

EXAMPLE 4 In this example, a chromium-plating bath may be made up containing the following ingredients: 22.8 parts of chromic acid, 0.72 of strontium sulfate, 1.50 parts of potassium silicofiuoride, 3.15 parts of potassium dichromate, and 0.63 parts of strontium chromate, in 100 parts total of aqueous solution.

A lead plating anode may be inserted in the bath together with steel-plating cathodes. The bath may contain a foraminous titanium basket or sleeve at one end thereof near the cathodes to be plated. The basket may be generally of cylindrical shape and 75 cm. long and have 1 cm. diameter holes therein more or less evenly spaced along the entire surface of a l-cm. diameter envelope and may form an envelope of the electrolyzing electrodes. Within the envelope formed by the titanium basket, there may be mounted an electrolyzing steel cathode 2.5 cm. X 75 cm. having a surface area of 1.9 square decimeters. The surface of this cathode may have been treated by immersion for to 30 seconds in a 1,000 parts total of an aqueous solution containing 4 parts palladium chloride, parts sodium chloride and hydrochloric acid sufficient to yield a pH of 1.5. Also mounted within the titanium sleeve may be two electrolyzing lead anodes 7.5 cm. X 75 cm. having a total surface area of 1 1.25 square decimeters.

Analysis of the chromium plating solution at the beginning ofthe experiment may indicate that it contains 22.8 parts (228 g./l.) of chromic acid. Chromium plating may be carried out in this chromium-plating bath by passing a current between the plating anode and plating cathode sufficient to yield a cathode current density of 45 asd. Chromium plating may be carried out for 8 hours at a temperature of 55 C. it may be found in this instance that after 8 hours the concentration of chromic acid may be 21.3 parts (213 g./1.) and the concentration of trivalent chromium ion may be 0.63 parts (6.3 g./1.). in a duplicate experiment the system may be maintained precisely as indicated except that a current is passed between the electrolyzing cathode and the electrolyzing anodes by means of a source of electrolyzing current sufficient to yield an electrolyzing anode current density of 5 asd. Electroplating may be carried out by discharging chromium on the plating cathodes while the electrolyzing current passes between the electrolyzing cathode and electrolyzing anodes. After 8 hours electroplating may be stopped and the concentration of chromic acid in the bath may be found to be 22.4 parts (224 g./l.) and the concentration of trivalent chromium may be 0.16 parts 1.6 g./l.).

Thus, it is apparent that operation of a chromium plating bath containing the electrolyzing system herein set forth, permits maintenance of trivalent chromium at desirably low levels and thereby prevents the various defects which would arise if the concentration of trivalent chromium were permitted to rise above the preferred concentration limit of less than about 5 g./1.

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications as come within the scope of the appended claims.

1. A process of treating a chromium-plating bath containing trivalent chromium to convert said trivalent chromium to hexavalent chromium comprising maintaining a plating anode and a plating cathode in a bath containing hexavalent chromium; plating said plating cathode by passing a plating current between said plating anode and said plating cathode whereby chromium is deposited onto said plating cathode and trivalent chromium is formed in said bath; maintaining in said bath an electrolyzing cathode of predetermined area the surface of which is in intimate electrical contact with a material in low hydrogen overvoltage state selected from the group consisting of etched iron compound prepared by action of acid on a material containing iron carbide, iron nitride or iron phosphide; platinum; palladium; rhodium; iridium; and nickel and an electrolyzing anode having a surface area at least equal to the area of said electrolyzing cathode; wherein said electrolyzing anode and said electrolyzing cathode are separated from said plating anode and said plating cathode by a porous partition and electrolyzing said bath between said electrolyzing cathode and said electrolyzing anode at an electrolyzing anode current density of at least 0.3 amperes per square decimeter whereby said trivalent chromium in said bath is converted to said hexavalent chromium.

2. A process for treating a bath containing trivalent chromium to convert said trivalent chromium to hexavalent chromium comprising maintaining in said bath and electrolyzing cathode of predetermined area composed of steel bearing a surface layer of palladium in a low hydrogen overvoltage state; maintaining in said bath an electrolyzing anode having a surface area at least equal to the area of said electrolyzing cathode; and electrolyzing said bath between said electrolyzing cathode and said electrolyzing anode at an electrolyzing anode current density of at least 0.3 amperes per square decimeter, whereby said trivalent chromium in said bath is converted to hexavalent chromium.

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2. A process for treating a bath containing trivalent chromium to convert said trivalent chromium to hexavalent chromium comprising maintaining in said bath an electrolyzing cathode of predetermined area composed of steel bearing a surface layer of palladium in a low hydrogen overvoltage state; maintaining in said bath an electrolyzing anode having a surface area at least equal to the area of said electrolyzing cathode; and electrolyzing said bath between said electrolyzing cathode and said electrolyzing anode at an electrolyzing anode current density of at least 0.3 amperes per square decimeter, whereby said trivalent chromium in said bath is converted to hexavalent chromium. 